A Growing Concern

After a few months of pondering how to build a simple pull-up vertical collinear antenna, I think I’ve finally got it! The motivation for such a design is the fact that I have tall trees for a support and can actually pull such an antenna up into the trees to 100 ft. or even higher. We all know that higher is better – and that is true with vertical antennas as well as horizontal dipoles.

The collinear design is simply two in-phase half-wave dipoles place end to end with a separation between the ends. This is simple enough in principle, but somehow you must feed each of the two half-wave structures with the proper phase while not having your feed lines short out the fields you are trying to produce. My first attempts fed the lower dipole via a co-ax line and I then tried to build a resonant structure that would send the wave up the wire to the top part of the antenna, phased with an inductor or a zig-zag delay line phase structure between the two halves. Unfortunately, these designs were difficult to model convincingly. The biggest problem is with the inductor, which is doing double duty as defining the ends of the radiating half-wave sections and as the phasing element. There still may be hope for these all-wire designs, but for now it was time to move on to something more theoretically tractable.

I decided to try for a design that deliberately fed the upper section with a coaxial feed line. The design I ended up with is elegant, robust to small dimensional changes, and offers very good theoretical performance. The figure below illustrates the design process (click to enlarge).

The vertical collinear antenna design transformed from the theoretical pair of phased dipoles a), into a pair of dipoles driven in series from a single source and phased with a section of transmission line b), to finally c), the practical design that uses the outer conductor of the feed lines as the lower three radiating elements of the antenna.

The collinear antenna gets its increased gain by doubling the number of half-wave active elements, effectively increasing the antenna aperture. For this to work, the two elements must work together in phase. One way to achieve this result is shown in b) above, where one element of the antenna is driven “in series” with the transmission line one wavelength long that then drives the second dipole section. Free space dipoles have a driving impedance of about 73 Ω when fed on center. If we use 75Ω line between the two elements, then the impedance and phase at the two ends of that line will look identical. The driving source will see the 73Ω impedance of the lower element in series with the transmission line impedance of 75Ω (which is well terminated by the 73Ω load of the upper dipole), hence about 150Ω is presented as the load to the driving transmission line.

Transmission lines have the property that the currents are balanced along the line. A coaxial line illustrate this best. The current coming out of the center conductor is balanced by an equal and opposite current traveling on the inside of the outer conductor. All of the fields, both electric and magnetic, are confined in the dielectric between the center conductor and the outer conductor of the cable. This happens because the mutual inductance of the two conductors of the transmission line is large and because, for RF frequencies, the skin effect prevents the fields from leaking out of the shielding conductor. If you look at the inset in the diagram above, you can see how the balance currents in the feed transmission line must be identical as the currents injected into the transmission line to the top section, and also identical to the current magnitude on the lower half-wave dipole.

The practical implementation, c) above, uses the outside of the coax shield as the dipole half-wave elements. The feed current, coming up the coax from the source encounters the break in the coax shield at the feed point for the lower dipole. For the currents to be continuous at this juncture, the current on the inside of the coax shield must turn the corner and go back down the outside of the shield. The center conductor current continues into the center conductor of the transmission line to the top dipole, and must be balanced by the current on the inside of this coax’s shield. But for the current to be continuous at the break, again we are forced to have a current in the outside of this transmission line’s shield as it turns the corner into the cable.

Once you grock the basic concept, that the break in the coax shield forces the currents onto the outside of the shield, it’s just a matter defining an effective length of the outer sections of coax that are to be radiating elements. We do this with resonant LC traps; the simplest version is just a coil of the coax itself to form the inductor L which resonates with the self capacitance C of the coil structure. When it comes to designing the traps, you may benefit from my attempt to distill some numbers in this Self Resonant Coil Calculator. At this point we have to compute the actual lengths of the elements and cable sections. Keep in mind that the electrical length of the coaxial cable will be different inside and outside. For the transmission line properties we need to use the velocity factor of the cable to determine the correct electrical length, while for the radiating elements the wave velocity will be close to the speed of light, modified a few percent by any dielectric jacket on the coax.

The table below lists the parameters that we come up with using this conceptual starting point in the 1st Cut column. There are a number of competing constraints that we have to deal with. The natural impedance of the series fed collinear will be about 150 Ω and we have to match to the 50 Ω source. The length of the coaxial feed section is barely long enough to include the wound trap coils and the radiating sections of the dipoles for there to be any coax for separation between the top and bottom sections. The free-space collinear antenna will get maximum gain when there is about 0.4λ spacing between the tips of the dipoles.

One method of addressing the matching problem would be to use an odd quarter wave length section of transmission line and transform the impedance from the top section to how it would appear at the bottom feed point. With the quarter-wave sections the wave-guide is ana-reflective and gets a perfect match when the impedance of the line is the geometric mean of the terminating impedances. Starting from the 50Ω source with 75Ω line, the perfect match at the antenna end would be 112.5Ω. That must be the impedance of the lower dipole, ~73Ω, in series with the apparent impedance of the line to the top element, which means we would like that to be 112.5 – 73 = 39.5 Ω. We could get that apparent impedance if the we again use the odd quarter wave trick and arrange to have the top feed point have an impedance of 752/39.5 = 142Ω. That might be possible by feeding the top section off-center. The problem with all of this is, of course, that the perfect 360° phase match for top and bottom sections will no longer be correct if we use the odd quarter wave line.

This is where optimization comes in. There are many variables, several paths to improved performance, but no definitive way forward.

Optimized 20 m collinear design

Design Parameter

1st Cut

Optimized

Units

Design Frequency

14.1

14.1

MHz

Design Wavelength

21.27

21.27

meters

Coax Type

RG-6

RG-6

Coax Velocity Factor

0.83

0.83

Coax Impedance

75

75

Ω

Height

30

30

meters

Length of line between top & bottom dipoles

17.65

23.67

meters

1.0

1.34

wavelength

L1 Length of top ¼ wave section

4.94

6.52

meters

L2 Length of top lower ¼ wave

4.94

2.84

meters

L3 Length of bottom upper ¼ wave

4.94

4.70

meters

L4 Length of bottom ¼ wave section

4.94

4.70

meters

Length of coax for each trap

3.6

5.1

meters

Trap coil diameter

256

196

mm

Trap turns

4.5

8.5

turns

Trap L

9.7

18.2

µH

Trap C

13.0

7.0

pF

Separation between dipole ends

0.5

5.93

meter

Model Results

SWR

1.78

1.09

Gain at 10° elevation

2.88

4.18

dBi

Radiative Efficiency

44.9

57.2

%

The results of the optimization process significantly lengthen the connecting coax section and generate an off-center feed for the upper section as suggested above. Now, with the two sections no longer perfectly in phase, not as much energy is aimed at the ground plane on the horizon so the overall radiative efficiency of the antenna has increased, and the vertical pattern has broadened to 30° elevation.

Antenna pattern and current for optimized collinear antenna.

This is all looking good. The 4NEC2 model is here if you wish to play with it yourself. The model included the effects of the insulating jacket on the coax and the THHN top section, so I hope the numbers are close enough to have good results on the first build. Practically, it will be good to understand the sensitivity of the design to the exact lengths of the various sections and how one could finally tune the antenna once it is ready to be deployed. Tuning the lengths of the radiating sections may be accomplished by rolling the inductor traps a few inches up or down the feed lines. I deliberately miss-tuned the bottom section (L4) short by 0.15 m and then ran a sweep on the other quarter wave section, L3, to see if the tune could be recovered. The figure below shows indeed it is possible to recover the tune this way.

Tune curve for the length of L3 after L4 was deliberately miss-tuned.

How does this design compare to other 20 meter vertical antennas? The table below shows a comparison of what you might expect with other architectures. There are a number of commercial verticals that probably fit somewhere between the 30 m high elevated dipole and the 5/8 wave ground plane antenna. The additional 4 dB in the low elevation DX window compared to the ground plane antenna is not to be sneezed at. There is no magic to be found that going higher and adding the second collinear element can’t beat for a vertical design.

20 m Vertical Antenna Type

Gain at 10° elevation (dBi)

Gain at 30° elevation (dBi)

Radiative Efficiency (%)

Optimized Vertical Collinear 30 m high

4.18

1.14

57

½ Wave Elevated Dipole 30 m high

2.94

1.39

57

¼ wave Ground plane with 64 radials 10 m long

0.29

2.72

41

5/8 Wave Ground plane with 64 radials 10 m long

1.1

-0.13

44

Speaking of going higher (or lower), the figure below shows how the gain varies with the antenna top height. Going higher gets even a bit more gain.

Gain at 10° elevation -vs- top height of the antenna (meters).

What does all this performance cost? Looks like you can buy a 1000′ spool of RG-6 on E-bay for less than $100. Be sure to get the stuff with foam polyethylene dielectric with 0.83 velocity factor. You need a couple of flower pots for coil forms, a scrap of wire for the top section, and some rope to pull the thing up into your tree. And you need that tree! How it all works out will be in another post once I actually build the beast.

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The last time the moon cast its shadow on my part of the planet was in 1979 on a dreary day in Seattle. I noticed that the rain was darker that morning, but otherwise the occasion was uneventful. This time around Ellen managed an invitation to watch the ecplise and make music with some friends in the country near Salem. It was the perfect location for viewing the eclipse with a sloping view to the east, and it came along with an awsome potluck feast and music making into the night hours Sunday evening.

Before setting out, I spent a few minutes with some optical parts from our kit at work so that I could take some pictures when the time came. We had a Cannon EOS M large sensor “mirrorless” camera which I fitted with a 400 mm focal length simple achromatic fixed focus lens. I found a few neutral density filters and tested the rig on the sun to make sure I could actually get images. Here are few of the nicer images I was able to obtain during the eclipse. Click on them to fill your screen with awesome totality.

Antenna patterns are all about interference

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The IC-745, like the older ICOMs, is built very robustly with parts that you can see without a magnifying glass, and which you can check with standard meters. The schematics are available on-line, the old ICOMs come up for sale on e-bay regularly, and there are support groups on line to help out when problems arise. The major problems are know and the fixes are documented. Although the IC-751 has the advantage of serial computer control that the IC-745 does not have, the IC-745 can also be a very good first HF radio for those willing to do a little restoration. These radios made in the early 1980’s are some of the first all solid state transceivers and represented state of the art technology when they came out. To this day, they perform well for the functions that they were designed for, mainly SSB and CW communications. However, the ham world changed in the last thirty years with the advent of the PC and the digital modes.

There are a few things that you need to accomplish to have the best possible operation on digital modes with the old radios.

Computer control of the radio (CAT). Sorry, we have to give that one up on the IC-745. It just was not made with serial control in mind. Arduino freaks could conceivably make that work – but you would be taking on a giant project.

Sound Card connections. Getting audio to and from the transceiver is required for digital modes.

Ensure stability and true frequency calibration. The narrow band digital modes care more about frequency shifts and drifts of a few Hertz which voice communications would never have noticed.

Extras like an RTL-SDR panadapter can provide modern functions to vintage radios.

Having given up on #1 lets get on to the others.

Sound Card Connection for the IC-745

Schematic section showing the ACC connector

To work the digital modes you need to be able to connect signals to and from a computer sound card. It is always possible to use the MIC connector, but then you will be always swapping out this connection when going between voice and digital modes. Better is to use the rear 24 pin ACCessory connector. The audio output from the IC-745 can be found on Pin-4 of the ACC connector. Ground is on Pin-8 next door. I directly wire an audio cable from Pin-4 to the sound card Line IN. It is a little harder to figure out where to put the sound card Line OUT. This transceiver was never designed to have anything other than a mic input, so we will need to play tricks. The MODE wire, Pin-5, is close to what we want. If you follow it, it comes from the one side of the MIC gain potentiometer. That seems promising until you realize that this point is the output of the mic preamp (wire MIC1). My solution was to sum the mic preamp and the MODE wire together through resistors at the high side of the mic gain pot. There is already a resistor on the MODE wire, R75, but none on the MIC1 wire.

If you stare at the bits of schematic I’ve provided, you can figure out this logic. I added a 1.5kΩ summing resistor to the MIC1 wire at the J8 header. This allows the MODE wire to be an audio input controlled by the MIC Gain knob yet still allows signals from the MIC preamp to also be used. The photo below shows how this was done on the J8 header.

With this simple modification, the ACC connector Pin-5 will work as the audio input form the sound card Line OUT. The VOX will also work using the sound card audio to key the rig. The MIC connector is free for the mic. Just be aware that the ICOM mic is always live, so if you do leave it plugged in, keep quite when transmitting via the PC sound card.

Resistor added to MIC1 wire on J8

Improving Frequency Stability on the IC-745

These early ICOM radios use a phase locked loop (PLL) stabilized VFO, which was state of the art when they were made. Unfortunately, the PLL circuits used some plastic trim capacitors which are guaranteed to deteriorate over time. They need to be changed out if they haven’t been already. A very good description of the trimmer replacement and other useful mods and adjustments written by Frank of W3UHF can be found here. Even with the trimmers replaced, the IC-745 suffers a small problem caused by thermal changes near the crystals that control the master oscillators in the radio. The problem is worse with radios with the internal AC power supply. The issue is that when the fan comes on to cool the output transistors, air is pulled in the vents in the bottom of the enclosure and the cool air flows over the crystals. They change frequency just a little. When the fans stops, the crystals warm up again and the frequency drifts back the other way. I found that isolating the crystals from the air flow with some Styrofoam “peanuts” help quite a bit. There are two crystals to protect for the 1st and 2nd local oscillators. I still see a little general temperature drift, but no longer on the ~1 minute time scale that made it difficult to hold a digital QSO.

Foam peanuts wedged over the LO crystals

The architecture of this radio makes it difficult for the TX and RX frequencies to be spot on. This is mainly an issue on the PSK31 mode where being off by 10Hz will mean you may have difficulty making a contact. I did the best I could with the LO frequency adjustments, but finally just resorted to using the RIT control to get things perfect.

RTL-SDR Panadapter pick-off point

1st IF Mixer and Filters. Pick-off point across R20.

RTL-SDR dongle pick-off

The 70 MHz first IF frequency on the IC-745 provides the perfect place to grab a signal for the ubiquitous RTL-SDR dongles that are available. With the RTL-SDR you can quickly see all of the signals on the entire ham band to which you are tuned. A little study of the schematic shows the appropriate pick-off point. You want to be right after the 70 MHz mixer but before the IF filters so that the dongle can see a wide bandwidth. In the schematic fragment, the point I chose was across resistor R20. I didn’t want to disrupt the circuit, yet need to get signal into a piece of co-ax, so opted for a 100 pf and 1.5kΩ series coupling into the 50Ω co-ax. I did not see any deterioration of the IC-745 signal doing this. In the photo you can see capacitor and resistor tacked in. The sub miniature coax, since moved over to my IC-751A, found ground on the bent over leg of R20.

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The 30 year old ICOM HF radios still have a bit of life in them if given the chance. I’ve managed to work all of the states and more than 140 countries using the digital modes with my old IC-751A in the last couple of years. A few small modifications have helped to run digital modes with this radio. This post will describe in detail the physical modifications and additions that I’ve done to the IC-751A to use it with a computer running FLDIGI or similar digital-mode programs. These mods should work for the IC-751 as well.

Sound Card Audio Connection: Without modifying the radio, you have two choices. You can use the rear panel auxiliary connector, Pin-5 for audio input, but you get no front panel knob to control the input level, nor can you use the VOX for PTT control. Or you can use the mic connector, but then you will always be disconnecting / reconnecting when you want to switch between the microphone for phone and the computer for digital modes.

Aux audio connection rerouted to VOX pot.

Two-pin connector for audio re-route

For these reasons, I made a small modification to the 751A wiring that allows use of the rear connector, but plumbs the audio input signal through the MIC GAIN control and VOX circuits. The pictures show the plan. The wire from Aux Pin-5, the trace labeled MO, is lifted from connector J5 and attached to trace VOXG on J7. I did this with the aid of a two-pin connector so I could easily back up if I needed to. As the picture shows, I picked off the ground from the shielded but ungrounded VOXG cable and also tapped off the connection at J7 Pin 1 for the two-pin connector for the MO wire. The last picture shows the MO wire rerouted to the two pin connector, ready to be covered up with shrink tube.

Wire with red sleeve is MO wire from AUX pin 5

Wiring re-routed

Wired this way, both the VOX GAIN and the MIC GAIN controls will affect the level of the audio input signal. Using the VOX is a handy way to key the rig when transmitting using the digital modes, so you should turn the VOX GAIN up enough that the digital audio signals from the PC reliably trip the VOX and key the transmitter. Then you can use the MIC GAIN control to fine tune the audio level into the transmitter. For many digital modes it is important to keep the RF amplifier linear and not overdrive it. I achieve this by leaving the RF PWR level control turned up all the way while using the MIC level control to then set the power output. If the rig wants to throttle your output, it will do so by raising the ALC level. This can happen if the SWR match is poor, or if you try to drive the output beyond the capability of the rig or beyond the RF power setting. Ideally, the ALC level reading on the meter should never lift off of zero for clean digital transmissions. But this means watching your output power level and using the MIC GAIN knob to adjust the input level, rather than throttling with the RF PWR control.

To complete the connections to your sound card, you will need to wire up the rear panel 24 pin auxiliary connector. You should connect “Line-IN” of the sound card to the AF OUT on AUX pin 4 and the “Line-OUT” on the sound card to the MO line on AUX Pin 5, using the common ground, AUX Pin 8, for both signals.

Serial Interface: If your radio doesn’t have one, get one! The good news is that Piexx makes a replacement for the long discontinued ICOM UX-14 option that is better than the original.

PTT from Piexx board

Piexx UX-14px board with PTT wire (purple) routed to top side of chassis

The new Piexx boards provide CAT controlled PTT and support reading the S-Meter. I’ve modified the FLDIGI rigcat IC-751.xml file to take advantage of these capabilities. If you wish to use the Piexx board’s PTT function, you can easily tap into the rig’s PTT line. The PTT line is called SEND on the schematic and can be found at J8 Pins 4 & 5. (You can find J8 on the schematic fragment shown in the picture above.) The pictures show the connection to the J8 header and wire routing. The S-meter connection is good to install as well, especially if you ever contemplate running your old rig remotely, which you could conceivably do with the serial connection and a remote desktop application.

RTL-SDR Dongle Connection: To add some real additional functionality to this old rig, attaching a $25 RTL-SDR dongle on the IF has to be the biggest bang for the buck that you can get.

RF section schematic showing SCOP connection

The SCOP connector J4 on the RF board

Coax routed from RF box and through the bottom cover

It is easy to do with the IC-751A. Just find the SCOP connector point and attach a piece of coax to go into the antenna port of the dongle. The pictures tell the story. There is a nice hole in the bottom cover that lets you get the cable out.

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You never know when you might need to measure the speed of light. When the time comes, this avalanche transistor pulse circuit will come in real handy. This is one of my favorite little bits of electronics because you get such great performance out of a single common 5¢ transistor that is abused for this purpose! Many bipolar transistors will exhibit avalanche breakdown if the voltage gets high enough on the collector, but you won’t find many that give a much better pulse than the common 2n3904.

Avalanch transistor waveform with ~2 ns FWHM

Timing pulse returned from open end of a 20 ft. length of coaxial cable.

The schematic shows the circuit. I built my version of this pulser into an existing timing box I had made before. You need a modest DC voltage to make the transistor avalanche so I used one of the primary windings on a small AC line transformer as the source for the ~150V DC supply. The charging resistor, R1, limits the current to the transistor. Above about 115V the 2n3904 will start to conduct, but will be quite happy as long as the current is limited by R1 to a few tens μA. A small 1:1 ferrite core transformer of a few turns works well for triggering the transistor. The base-emitter junction is turned off hard by the DC short of the transformer. When triggered, the transistor undergoes an avalanche breakdown with < 1 ns rise-time, discharging C2 through the emitter resistor load. You can trigger at >1kHz rep-rate without the pulse losing amplitude. I arrange the load to tap off a fraction of the discharge voltage and also to appear as a matched 50Ω load to absorb any reflected pulse returning on the cable. The discharge capacitor, C2, can either be a small, few pF cap, or it can be a short piece of transmission line if you want a square pulse. I like the spike, so went with 22 pF into about 50Ω which discharges the capacitor in about 1 ns when the transistor switches. The waveform I see on my scope with 400 MHz bandwidth is a smoothed pulse about 2 ns wide into a 50Ω load. If I had a faster scope it would be even sharper.

What good is it? Well, if you are after a low-jitter fast-rising edge trigger, you can’t beat that spike. The pulse is also very handy to time cables, to adjust them for identical or specific electrical lengths, or just to find out how long a piece of cable actually is without getting out a tape measure. The second oscilloscope trace shows the unmistakable reflection from the open end of a length of coaxial cable. The measurement functions on the scope says that it took 61.2 ns for the pulse to traverse the cable and return. RG-58 cable with polyethylene dielectric has a velocity factor 0.66 the speed of light. Hence the length of cable can be calculated as: L = 61.2 ns / 2 * 3.0 x 108 * 0.66 = 6.06 m = 19.9 ft. which is pretty close to the length I measured at 19 ft. 8 in.

If you get out your nippers, you will find that you can trim cable lengths to within just a couple of inches of the length you want without need of a tape measure. I find it pretty amazing that we can tell how long it takes light to travel over just a couple of inches of wire… Along with a decent oscilloscope, a fast pulser makes all the difference for this kind of measurement.

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I have had a chance to use HDSDR a little more with the RTL-SDR pan-adapter for my IC-751A transceiver and have found it a very nice tool for a number of applications. Here are a few examples.

Pan-adapter for rapid SSB tuning

This is the obvious application. Using the RTL pan-adapter with HDSDR set to “Full sync in both directions,” it is possible to monitor the entire ham band for SSB signals and to tune the parent radio to the transmission by clicking on the signal in HDSDR window. The received signal can be heard either through the parent radio or demodulated with HDSDR. Either the RTL-SDR or the conventional receiver might sound better, depending on the quality of the parent receiver. The filter edges are very steep and simple to adjust in HDSDR, and there is a very sharp notch filter that can remove unwanted narrow-band signals that might be contaminating the voice channel. Nevertheless, to my ears the wider audio notch in the 751A seems to provide more readable and comfortable voice signals.

The HDSDR screen shot above shows more than a dozen voice signals to sample across the 20 meter band. One of the nice features of this program is the easy way to set the squelch with a click on the S-meter. A curious digital mode transmission is in progress just above the ham band at 14.366 MHz. For the voice modes, the frequency synchronization and stability between the parent receiver and the RTL-SDR radio is plenty good enough that your transmitted signal will fall on top of the HDSDR frequency settings (once you have it all set up correctly) if you are listening using HDSDR.

RFI identification and signatures

The big view of the pan-adapter can show more than just intentional transmissions. In the screen shot below you can see the situation on 30m this afternoon. The wide view makes RFI problems unequivocally show their presence. Below you see a strange on/off modulated RFI signature every ~17kHz across the band. RFI can come in many forms. Switching power supplies can generate bands of noise spaced at uniform frequency intervals. One of the next projects here will be to set up a portable RTL-SDR computer rig for chasing down such RFI sources, which can plague reception especially on the lower bands in my neighborhood.

Working the pile-up

To the left of the center of the waterfall above you can see a pack of CW operators in a “pile-up”, all looking to contact a rare DX operator. In this case a “DX expedition” to the South Sandwich Islands is the attraction. Breaking through the pile-up is an art that I have yet to master, but I can see that the wide view of the pan-adapter can be invaluable to help identify the actual station of interest in the midst of the hoards of other signals seeking attention, as well as the frequency where the operator is listening by identifying where in the pack the selected stations are replying. If the DX station is working “split,” outside the 3kHz bandwidth of the 751A, picking the correct place to call can be purely guess-work without the wide-band knowledge.

Monitor SSB net while working PSK on the parent radio

The are many examples of “doing two things at once” that can be accomplished because of the ability of HDSDR to be tuned independently of the parent receiver. For starters, tune the transceiver to the PSK portion of the band. Select independent tuning for HDSDR. Configure the audio output from HDSDR to the computer speakers and go hunting on the voice section of the band. Listen in on your friends while you use FLDIGI to work the PSK section of the band with the parent transceiver.

Monitor the JT65 & JT9 band while working PSK – or vise versa

It is possible to run two digital-mode decoding programs, one for the parent transceiver and one for the RTL-SDR receiver tuned through HDSDR. Yes, you will have three radio programs going at once! Sometimes it is not possible to get everyone to CAT correctly – so here is my general plan. The working application will be connected to the transceiver audio in the conventional manner, and will be controlled from HDSDR using the “CAT to HDSDR” port via a Virtual Serial Port (VSP). The monitor application will get it’s audio from HDSDR via a Virtual Audio Cable. You will have to manually tune this monitor application to match the HDSDR tuning since the working applications is using the “CAT to HDSDR” port. Use “independent tuning” so that the HDSDR’s LO control tunes the transceiver and the working application, while HDSDR’s Tune control tunes the monitor application. Below are step-by-step instructions for setting this up:

Configure HDSDR and Omni-Rig to CAT to the transceiver.

Set up a Virtual Serial Port between “CAT to HDSDR” and working program you will be using to work contacts, e.g. FLDIGI.

Set that program up to accept VSP connection as a Kenwood TS-50 as HDSDR requires.

Test that you can control the rig and HDSDR via the working program, FLDIGI for example. Changes from the rig will reflect back to HDSDR which will then make the changes to FLDIGI. Similarly going the other way, HDSDR is in the middle.

Set HDSDR sync mode to “independent tune in HDSDR.” Note that tuning the LO frequency control will change the transceiver frequency.

Bring up the monitor application, e,g. WSJT-X. Set up for no CAT control.

Connect the monitor application to HDSDR audio output using a Virtual Audio Cable.

Tune HDSDR to the band section you wish to monitor. Manually tune the monitor application to the same frequency.

Go back to the working application and make some contacts. You can tune around the band with the working application all the while the monitor application remains at the desired frequency. However, if you change bands you will have to re-tune the monitor application.

I almost always try to work stations with audio routed to/from the transceiver in the conventional manner. It is possible to listen to the RTL-SDR / HDSDR demodulated audio, and then transmit using the parent transceiver. However, my RTL-SDR dongles do not have the frequency stability that allows me to know for sure that I will be exactly on-frequency. A frequency shift of just a few Hz between Rx and Tx can lead to confusion with digital modes. Using “split” modes can help deal with Rx/Tx discrepancies.

You can use the set-up above as a much more complete digital-mode band monitor. Set the applications, e.g. FLDIGI and WSJT-X to report spots to the PSK Reporter website. JT65 and JT9 spots are the most productive, but there are plenty of PSK signals as well that might be of interest that FLDIGI can flag. Once your spots are logged to PSK Reporter, you can quickly see what parts of the world you are hearing via the PSK Reporter website map.

I’ve also used RCKskimmer, to monitor the band for RTTY and PSK31 & PSK63 signals. If RCKskimmer is looking at the RTL-SDR radio tuned through HDSDR, then you can skim signals from the PSK and RTTY sections of the band. Meanwhile, the transceiver can be tuned on the JT65 region. Again, all spots can be sent to PSK Reporter.

Using these techniques you might be amazed at what your antenna is picking up when you otherwise are not actively listening. I’ve started using WSJT-X 1.70 recently which has a much better decoder than previously. I’ve seen more than two dozen signals simultaneously decoded when looking at the full JT band with the RTL-SDR using the new WSJT-X. The screen shot below shows typical activity seen on 20m over a few hours monitoring both the entire JT65 & JT9 bands and the PSK band. If I keep the radio on I reliably end up in the top 20 monitors in the PSK Reporter statistics despite my out-of-the-way location in the Pacific Northwest.